1. Field of the Invention
This invention relates in general methods and apparatus for producing screen printing forms where the engraving is accomplished with small cups or depressions arranged in a printing screen and the engraving device is driven by an image signal and a screen signal to produce a printing screen and the engraving needle of the engraving means receives periodically recurrenting amplitude variations in the screen signal to produce its largest deflection in the direction of the printing form and where analog image signals obtained by pattern scanning are converted to digital signals at a frequency of the screen signal and subsequently, converted from digital back to analog signals and superimposed for driving the engraving means.
2. Description of the Prior Art
U.S. Pat. No. 4,013,829 describes an engraving machine for producing screen printing forms from patterns. The patterns may be continuous tone patterns, so-called line patterns with writings and line representations or combinations of both.
The pattern to be reproduced which is clamped on a rotary scanning cylinder is scanned by scanning means driven parallel to the axis of the scanning cylinder such that the scanning occurs dot by dot and line by line. Depending on the tonal values of the scanned image points, more or less light is reflected from the pattern into the scanning means and converted there opto-electronically into an image electrical signal which is then converted into a digital signal by means of a scanning cycle sequence.
Subsequently, the digital image signal is modified according to a predetermined logarithm and/or gradation curve by recording with the aid of a programmable read only memory (PROM) such that the gradation curve is oriented according to the subsequent printing process and the desired editorial changes of the reproduction relative to the original.
The modified digital image signal then is reconverted into an analog image signal and supplied to an electro-magnetic engraving means which has an engraving pin that serves as the cutting tool said engraving means moves axially along a printing cylinder so as to engrave the cups point by point and line by line.
During the engraving of the printing form, the engraving means engraves a sequence of cups or depressions arranged seamlessly in a printing screen and the depth of the depressions or cups depend in each case on the tonal value of the associated image dot.
For the actual screening, a screen signal whose frequency depends on the printing screen to be recorded and the surface speed of the printing cylinder is superimposed on the analog image signal.
While the screen signal generates a vibrating cutting motion of the engraving pin corresponding with the printing screen and determines the depth or moments for the engraving of the cups or depressions, the momentary image signal determines the depth of penetration of the engraving pin into the particular cup being formed.
The engraved printing cylinder forms the printing form for the later printing process in a photogravure rotary printing machine.
The exact reproduction of edges or sharp changes in tone density in the pattern represents a particular problem in the production of screen printing forms. In the engraving system described in U.S. Pat. No. 4,013,829, the positions of the cups on the printing form are predetermined by the position on the printing screens. On the other hand, while scanning the change in tone density lines extend arbitrarily with regard to the scanning screen which is congruent with the printing screen to be recorded. Fine courses of contours, therefore, can be reproduced only definitely with a rigid cup arrangement. For example, fine contours extending obliquely to the scanning direction illustrate disturbing staircase like formations.
Additionally, in the engraving system described in U.S. Pat. No. 4,013,829, the shade values in the range of changes in tone density are reproduced in a varied manner because of the mass of the electro-mechanical engraving means which factor is disadvantageous.
Sharper change in tone density reproduction could be accomplished using finer printing screens. However, a finer printing screen has the drawback that during subsequent printing processes, the transfer of ink from the cups to the printing medium is defective due to smaller cup volume.
U.S. Pat. No. 3,983,319 provides an improvement of the tone density changes by position shifting of screen points within the area of the change.
In the reproduction process for the exposure of screen images upon a photosensitive medium as described in U.S. Pat. No. 3,983,319 for the purpose of a sharper reproduction of the tone density changes those screen points which reproduce tone density changes are so shifted with regard to the positions predetermined by the printing screen that they substantially impinge upon the tone density change lines.
In this reproduction apparatus, patterns are scanned additionally with a photo diode device arranged transversely to the scanning direction. With the presence of a tone density change extending in the scanning direction in the pattern, the photo diode device produces a control signal to the recording apparatus. The control signal deflects a rotary mirror located in the path of the light rays of the luminous source used for the recording such that the screen points are exposed and shifted perpendicularly to the direction of recording.
This reproduction process is subjected to the disadvantage that only tone density change extending in the scanning direction can be reproduced in an improved manner. A precise determination of the location of a tone density change is not possible.
Another disadvantage of this system is that the method is not applicable for electro-magnetic engraving means because the engraving needles cannot be deflected rapidly either in a recording direction or transversely thereto, but merely performs an in and out stroke movement in the radial direction relative to the printing cylinder.